With a high three-quarters delivery Chen pitches a fastball that usually sits around 90 mph and tops out at 96 mph. The best breaking ball of his package is a slider. Chen commands his fastball well against batters, but his slider isn't that effective with common quality and control. In fact, Chen can be considered as a two-pitch starter in NPB.

The Chunichi Dragons signed Chen for a $850,000 bonus including incentives. He made his NPB debut in 2005 and was ineffective. In 10 games, he had a 6.05 ERA and a save, allowing 29 hits, 6 walks and 2 wild pitches in 19 1/3 innings, though he struck out 20. He was 1 for 3 at the plate. Chen did not pitch for Chunichi's top club in 2006 or 2007.

Chen pitched 7 shutout innings against the Dutch national team for a win in the 2008 Olympics, as Taiwan beat one of their closer rivals in recent events. He was less effective against Team Canada, giving up 4 runs in 3 1/3 IP in a no-decision as Taiwan topped Canada to clinch 5th place. Chen hurt his own cause, making a throwing error on a bunt by Emmanuel Garcia, allowing Adam Stern to score. Chen had the 4th-lowest ERA on the Taiwan staff in Beijing and got one of the two Taiwan wins.

The young left-hander was even better in 2009, going 8-4 with a 1.54 ERA. He struck out 146 in 164 innings and allowed only 113 hits for a .193 opponent average. He led the CL in ERA by .46 over Kazuki Yoshimi, though #3 Dicky Gonzalez beat out both of them for the Best Nine spot at pitcher. Chen tied Yoshimi for the CL lead in shutouts (4), tied for second in complete games (5) and was 5th in strikeouts (40 behind leader Lewis but only 8 back of #2 Atsushi Nohmi). He had his best season in 2010, with a record of 13-10, a 2.87 ERA and 153 strikeouts in 188 innings. In 2011, he posted a 2.68 ERA in 25 games for Chunichi, with a record of 8-10 but only 94 strikeouts. After the season, he asked for his release, which was granted, in order to test the international market.

On January 10, 2012, he signed a three-year deal with the Baltimore Orioles, with a club option for a fourth. The Orioles could afford to sign him for the long term as he did not need to go through the posting system, thus saving the club the need to come up with a posting fee. He was the second major signing of a pitcher from Japan by the O's that offseason, following that of Tsuyoshi Wada in December. He made his debuty with the Orioles on April 10th, getting the start against the New York Yankees; he gave up 4 runs (2 earned) on 7 hits in 5 2/3 innings and had a no-decision. He was one of the Orioles' most reliable starters over the first half, helping the team to be surprisingly competitve in the tough AL East. On July 29th, he set a record for Taiwanese pitchers by striking out 12 batters in 5 2/3 innings in a 6-1 win over the Oakland A's; Chien-Ming Wang held the previous record with 10 strikeouts in a game on June 17, 2007; they were also the most by an Orioles pitcher since Erik Bedard had struck out 15 on July 7, 2007. The win improved Wei-Yin's record to 9-6. He finished his rookie season at 12-11, 4.02, pitching 192 2/3 innings in 32 starts. He started and won Game 2 of the ALDS against the Mew York Yankees, giving up 2 runs (1 earned) in 6 2/3 innings in a 3-2 win.

Injuries limited Chen to 23 starts in 2013, but the results were very close to those of his rookie season, as he ended up with a record of 7-7, 4.07, pitching 137 innings. In 2014, he took a big step forward as he was the Birds' most-effective starter as they took over the lead in the AL East in early July. Following a win over the Seattle Mariners on August 1st, his record was 12-3 with a 3.76 ERA in 21 starts. He finished the season at 16-6, 3.54 in 31 starts, logging 185 2/3 innings and 136 strikeouts against 35 walks. He was hit hard in his Game 2 start in the ALDS against the Detroit Tigers, giving up 5 runs in 3 2/3 innings, but the Orioles came back to win the game, 7-6. He pitched better in Game 3 of the ALCS against the Kansas City Royals, giving up 2 runs in 5 1/3 innings, but was charged with his team's 2-1 loss.

Wins were hard to come by in the early going of the 2015 season. After 13 starts, his record was only 3-4, in spite of an ERA of 2.89, good for an ERA+ of 136. In his start of June 15th against the Philadelphia Phillies, he pitched 8 scoreless innings to earn a 4-0 win, his second straight winning start. It was thus a major surprise when after the game, the Orioles sent him down to the minor leagues. The Orioles' brass explained that he had "general soreness" which begged the question of why he was going to the minors and not on the disabled list in that case; anyway, Chen denied there was anything wrong with him. The decision may have been a way to avoid him making a start against the Toronto Blue Jays, a team which had absolutely destroyed left-handed pitching that season, but if it was truly the case, it seemed a very blunt way to proceed. His agent, Scott Boras, hinted that he would file a grievance.